As a conventional technique in the above-described field, there are techniques described in Patent document 1 and 2 listed below. Gas (deuterium) discharge tubes described in these patent documents both have a metal-made barrier on a discharge path between an anode and a cathode, and in this barrier, a small hole is formed so as to narrow the discharge path. In this construction, light with high luminance can be obtained by the small opening on the discharge path. Particularly, in the gas discharge tube described in Patent document 1, the small hole, that is, the portion to narrow the discharge path is lengthened to further increase the luminance. On the other hand, in the gas discharge tube described in Patent document 2, the length of the small hole is increased and a plurality of barriers are provided to make the luminance higher.    Patent document 1: Japanese Published Unexamined Patent Application No. H07-288106    Patent document 2: Japanese Published Unexamined Patent Application No. H10-64479